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Studying Our Rivers

How much water do our rivers and streams need? Texas scientists are looking for the answers.

By Kevin Mayes<

Every drop of water counts, but why it counts depends upon your perspective. A water purveyor might see a drop of water slipping past a dam as lost for human consumption. But that same drop of water traveling downstream creates habitat for wildlife and supports myriad recreational opportunities — hunting, fishing, tourism, bird watching and paddle sports. Water that runs down a stream may rest in a quiet pool before moving down through an alternating chain of gliding runs and bumpy riffles, which are often the most productive and richest habitats in the river.

When water jumps the cottonwood- and willow-lined banks of a stream during a flood, it carries with it sediment and nutrients that nourish bottomlands and other streamside wetland areas. These riparian areas serve as natural sponges, soaking up pollutants and excess nutrients. They also contribute to the diversity of the river ecosystem — riparian areas are known for their tremendous productivity and richness of species. Hunters target these areas for waterfowl, turkey and deer, while birdwatchers come to see woodpeckers, kingfishers, shorebirds, vireos and warblers.

During dry times, each drop of water counts even more. As creeks and small springs run dry, streams and rivers turn to trickles and the competition for space, food and oxygen heats up. Because they are the highest part of the streambed, riffles tend to dry up first. Water barely covers the cobbles and pebbles and offers little habitat for some species that can’t thrive anywhere else. Pools warm up and fish congregate at the tail of the riffle, waiting for the next meal to make a run for safer cover. Birds and other animals often find easy meals in isolated pools on the edge of the channel. In spite of these harsh conditions, stream organisms find havens and repopulate abandoned areas once the flows return. They are adapted to a natural cycle of dry and wet periods.

Drought is what worries river scientists the most. As the land parches, the need to irrigate crops and urban lawns intensifies; reservoir levels drop, and water purveyors become alarmed. Water diversions and reduced releases from storage reservoirs can prolong or exaggerate the effects of drought and aggravate impacts to fish and wildlife. With the population of Texas expected to nearly double in the next 50 years, from almost 21 million people in 2000 to about 40 million in 2050, the demand for water will certainly increase. The question is whether rivers and streams and the aquatic life they support can continue to recover from dry times and droughts in the face of ever-increasing water demand.

The challenge now is to find out how much water it takes to maintain healthy aquatic ecosystems. This is not an easy question because river ecosystems are complex and dynamic, being interconnected with groundwater, floodplains and estuaries, and characterized by interactions of hydrologic, physical, chemical and biological processes. For example, high flows move sediment, flush silt, build and reshape habitats and connect the river to the floodplain. Flowing water influences important water quality parameters such as temperature and oxygen levels. Land-use practices such as urbanization, agriculture and logging alter the quality and quantity of water by modifying watershed retention and runoff. Biological processes are also linked to hydrology. River biota have adapted their life histories to take advantage of the seasonal changes in flow, some timing their reproduction to high springtime flows and others migrating long distances upstream to complete their life cycle. Some fishes require different habitats at night, or change habitats as they mature or when they need to spawn. Some are specialists, using only certain habitats for all of their lives.

In 2001, recognizing the potential for serious conflicts between the need for water for human uses and for maintaining rivers and streams, the Texas Legislature directed the Texas Parks and Wildlife Department, the Texas Commission on Environmental Quality and the Texas Water Development Board to determine the flow conditions necessary for maintaining healthy streams and rivers in Texas. Those priority studies — termed instream flow studies — are to be completed by 2010. The three agencies jointly developed two documents that describe the priorities and the tools for conducting the studies. The Programmatic Work Plan for Texas Instream Flow Studies (December 2002) identifies timelines for six priority studies, outlines the roles of the state agencies and presents the scope of the studies along with the general methods used to conduct the studies. A supplementary document, the draft Technical Overview (August 2003), provides an in-depth technical discussion of the proposed science and engineering methods and identifies the conservation of biodiversity as well as the maintenance of biological integrity as study goals. In 2003, as part of the peer review process, the Texas Instream Flow Program sought a review of these two documents by the National Academy of Sciences. The Academy’s National Research Council assembled a multidisciplinary committee to perform the review, and it will be complete in October 2004.

The Texas Instream Flow Program can have an influential impact on the future of Texas’ natural heritage by providing accurate and useful data and tools for water planning, permitting and the conservation of fish and wildlife. More information about the program and the NAS review is located on the Texas Instream Flow Program website at www.twdb.state.tx.us/instreamflows/.